1. Field of the Invention
The present invention relates to a method for the pelletization of plastics and/or polymers, wherein a melt coming from a melt generator is supplied via a diverter valve having different operating positions to a plurality of pelletizing heads through which the melt is pelletized. The invention furthermore relates to a pelletizing apparatus for the pelletization of plastics and/or polymers having a diverter valve which has at least one melt generator connection, at least two pelletizer connections as well as a switching gate for selectively connecting the melt generator connection to at least one of the pelletizer connections, with a respective pelletizing head being connected to the at least two pelletizer connections and a melt generator having a variable melt volume flow being connected to the melt generator connection. Finally, the invention also relates to a diverter valve for such a pelletizing apparatus having a melt generator connection, a pelletizer connection as well as a melt passage for the connection of the melt generator connection to the pelletizer connection.
2. Description of the Related Art
As a rule, diverter valves via which the pelletizer is connected to the melt generator are used for the start-up of pelletizer devices. This in particular applies to complex production processes whose start-up procedure is difficult as well as to applications in which uniform pellets should be generated as rapidly as possible. Diverter valves of this type are described, for example, in DE 102 34 228 A1; DE 38 15 897 C2 or EP 0 698 461 B1. These diverter valves comprise, in the melt passage which connects the inlet opening of the valve at the melt generator connection to the outlet opening at the pelletizer connection, a diverter gate which interconnects the connection of the melt generator connection to the pelletizer connection in the production position, whereas it keeps the melt flow away from the outlet opening at the pelletizer connection in its start-up position, i.e. it blocks it and diverts the melt loss so that the melt flow entering at the melt generator connection does not move to the pelletizer connection, but instead exits at a bypass opening of the valve and as a rule simply flows onto the floor. If the pelletizer device has started up so that all the units are working with the desired operating parameters and the melt flow has reached the desired quality, the diverter gate is switched over to its production position so that the melt flow in the diverter valve. flows to its pelletizer connection and is then processed to pellets by the pelletizer connected there.
The start-up phase of a production process can admittedly be effected per se in a satisfactory manner using such known diverter valves; however, problems occur on the changing from one production process to a second production process, for example on a change of the polymer/filler mixture, on a change of the pellet geometry, on a changeover to changed throughput demands, on a change in the color of the pellets or also on scheduled or unscheduled production interruptions e.g. for repairs to the nozzle plate. The problem which results in this process is that the total diverter valve, including the melt passage in the interior of the valve, has to be cleaned completely before the plant can be started up again. Without such a cleaning, contaminations of long duration would occur, for example on the changeover from colored pellets to white pellets. Conventional diverter valves have to be dismantled for cleaning as a rule, whereby the production process is interrupted for a longer duration. Moreover, subsequent to the cleaning, the fitting time has to be taken into account which is needed, for example, for the heating of the diverter gate to operating temperature.
The possible alternative of having two separate diverter valves available for such changes between two production processes is not acceptable for a number of operators of such plant. On the one hand, the costs for two complete diverter valves are incurred. Apart from this, time delays also occur on the use of two separate diverter valves, e.g. due to the ramping up of the new diverter valve to operating temperature.
Furthermore, DE 696 21 101 T2 describes the possibility of viscosity change within a compounding process with subsequent pelletization in a corresponding large-production plant having a performance of at least 1000 kg/h. Two pelletizer heads are connected to the valve connected downstream of the melt generator so that highly viscous material can be given to the one pelletizing head and low viscosity material can be given to the other pelletizing head by switching over the valve. The problems of the start-up losses are, however, not solved in this process; it is rather the case that material not yet pelletizable should be discharged via a bypass opening in a manner known per se up to the reaching of the respective operating point. Furthermore, a pelletizing apparatus is described in DE 197 54 863 C2 in which two pelletizing heads are connected to a three-way valve so that, on a color change from black material to white material or vice versa, the one or the other pelletizing head can selectively be selected. To so-to-say flush out color contaminations on a color change in this process, a central bypass outlet is provided in the valve via which material of the new color is discharged for so long after a change of color in the melt generator until even the last contaminants have been taken along. This is more counter-productive than helpful with respect to the aforesaid objective of reducing start-up losses and of decreasing expensive material waste. Finally, a multiway rotary valve for pelletizing plant is known from DE 100 30 584 with whose help its high molecular plastic melts can be distributed or split up. The problems of the start-up losses are, however, also not addressed in this reference.
With a customary design of an underwater pelletizing plant, the start-up losses which occur and the corresponding material loss are definitely cost intensive. In particular with polymers or plastics sensitive to freezing, e.g. products with a high crystallite melting point, it is necessary to start and to operate at a minimal throughput of more than 10 kg/h per nozzle bore. After the actual starting process, the subsequent throughput increase is unproblematic as a rule. However, material losses arise due to the starting process itself due to start-up product in block form on the floor which can easily amount to several kilograms. This is not only uneconomical because the expensive raw materials are transferred in a non-sellable form, but is also unpleasant for the operator of a corresponding production plant since the blocks can turn out relatively large and have to be reduced to small particles in an expensive process and finally have to be disposed of. Such a hot melting block having temperatures of, optionally, more than 250° C., and discharged via the bypass outlet of the diverter valve not least also represents a potential safety risk. The problems of the discharge of plastic melt via the bypass outlet does not only occur in the actual start-up of a corresponding production plant for a new production job, but also when, for whatever different reasons, the plant has to be operated out of the throughput window of the pelletizing head, in particular when the melt volume flow has to be operated below the lower capacity limit of the respective pelletizer head. Here, too, the diverter valve sometimes has to be switched into the bypass position so that corresponding material waste arises.